Much of the middle stretch of the Border was protected by the Cheviot massive, although it was less of an obstacle to reiving bands.  Indeed it provided a variety of convenient routes and valuable cover.

The wide expanse of relatively low lying land from the Cheviots to the east coast was, of necessity, protected by a chain of formidable castles.  This was not only a popular raiding route but was the route frequently used by invading armies.

The castles at Wark on Tweed and at Norham were built soon after the Norman conquest.  Later, the massive castles at Alnwick, Warkworth, Bamburgh, Morpeth and, further south, a castle at Prudhoe on Tyne was added to the defensive system, the whole of which served to keep in check the restless Northumbrians as well as to repel Scots invaders.

Further defensive structures were progressively built to protect the scattered villages and farms of Northumberland and Cumberland were the peel towers, which were similar to but smaller than  the stone keeps of the castles. These peel towers were usually two or three storeys in height and had either an interior staircase or a removable ladder outside to the first floor. The ground floor entrance was protected by a heavy wooden door or yett strengthened by horizontally placed draw bars of timber. Additional protection was provided by an outer wooden or stone wall which formed an enclosure for livestock in time of danger.

The first floor provided temporary living accommodation and the upper floor sleeping quarters.  The roof provided a vantage point and was usually be equipped with a metal cradle for a fire or beacon to signal afar a warning of danger.

Bastles served at a lower level of protection. These were much smaller stone structures, quite strong, but unlikely to withstand a determined attack. The ground floor had a small entrance and was used for storing the more valuable possession, while the upper floor provided a reasonable level of safety for the family.  There was no permanent staircase but a removable ladder either inside or out.  The outside stone staircases seen on many existing bastles were added with the coming off more peaceful times.

Bastles were usually built in clusters within 20 miles of the Border. They are quite uncommon in Scotland.

It is interesting to note that no  buildings comparable to peel towers and bastles are to be found along the Welsh Borders.

A great arc of these defenses were built from Wark on Tweed in the north and then south east to the castle at Harbottle. The line then took a southerly direction taking in Tossan and then on to Prudhoe Castle. Further defenses were constructed westward into Cumberland. This system of fortresses should have provided adequate protection to the exposed borders of Northumberland but they were often neglected and allowed to fall in to disrepair situation which the Scottish reivers readily exploited.